1. Field of the Invention
The present invention relates to a method of adjusting the interstation delay in an information transmission system utilizing in one transmission direction the TDMA method (Time Division Multiple Access), the transmission system being constituted by a network comprising a central station and a plurality of sub-stations which includes relay sub-stations arranged in cascade, the sub-stations comprising time-shifting means for shifting the instants of transmitting the information bits to the central station, the central station comprising time-detecting means for detecting the order in which the said information bits arrive, each station in the network comprising at least a transmit time-base and a receive time-base.
2. Prior Art
A method of adjusting the interstation delay in such an information transmission system is described in the French Patent Specification No. 2,502,426 (corresponding with U.S. Pat. No. 4,472,802). In the information transmission system, the information signals are transmitted in accordance with the TDMA method in the transmission direction from the sub-station to the central station. A sub-station only transmits when a time slot has been assigned thereto and in this time slot just one single station is authorized to transmit. All sub-stations operate in synchronism with the central station and solving the problems as regards synchronization is essential to avoid, on reception at the central station, the overlapping of information signals originating from different sub-stations. To solve these synchronization problems the sub-stations comprise means for shifting the instants of transmitting the bits of their information signals, and the central station comprises means to detect whether the information bits arrive properly located the time.
In this manner the problems of synchronization are solved, but when a terminal sub-station at a subscriber is very remote from the central station, there is still one problem left to be solved, for it is then necessary to insert a large number of relay sub-stations in a cascade arrangement, relay sub-stations which also themselves can naturally be connected to terminal sub-stations at subscribers. Each relay sub-station has to recover the clock from the information bit stream coming from the central station and utilize this clock to retransmit in its turn, which in the case of a considerable number of relay sub-stations can produce phase-shifts leading to jumps of several bit periods. Actually, the reception of the bits of the information signals coming from the sub-stations situated further away takes place with the aid of recovering the clock from the received information bit stream, and the phase of this recovered clock with respect to that of the sub-station can have any value, for it is a function of the interstation distance and can change under the influence of specific phenomena such as temperature variations. If, in an initial configuration, the two clocks are substantially in-phase, a slight phase variation can translate into a shift over one bit period, and when many relay stations are connected in cascade, the shift can amount to many bit periods.